1. Field of the Invention
The present invention relates to golf balls. More particularly, the present invention relates to improvement of dimple of golf balls.
2. Description of the Related Art
Golf balls have numerous dimples on the surface thereof. The dimples cause turbulent flow separation through disrupting the air flow around the golf ball during the flight. By causing the turbulent flow separation, a separating point of air from the golf ball shifts backwards leading to the reduction of drag. The turbulent flow separation promotes the differentia between upper separating point and lower separating point of the golf ball, which results from the backspin, thereby enhancing a lift force that acts upon the golf ball. The reduction of drag and the enhancement of lift force are referred to as “dimple effect”. Excellent dimples disturb the air flow more efficiently. The excellent dimples achieve great flight distance.
For golf balls, not only flight distance but also aerodynamic symmetry is important. The flight distance of the golf balls with excellent aerodynamic symmetry does not depend on the rotation axis of the backspin. Golf players easily let this golf ball drop on a targeted position. Also, in view of conformity to the rule defined by United States Golf Association, aerodynamic symmetry is important.
A mold for a golf ball consists of an upper mold half and a lower mold half. Each of the upper mold half and the lower mold half has a hemispherical cavity face. When the upper and the lower mold are mated, a spherical cavity is formed. This mold has a parting line between the upper and the lower mold half. The parting line is a great circle. On a surface of a golf ball, a portion which corresponds to the parting line is referred as an equator (or a seam). The equator is a great circle. The vicinity of the equator is a particular region. U.S. Pat. No. 6,066,055 (JP-A No. 2000-93556) discloses a proposal to remove the particularity in the equator vicinity region.
In the vicinity of the equator, many of dimples tend to be arranged orderly. When golf balls have dimples arranged orderly, a great dimple effect can not be achieved. With this golf ball, a sufficient flight distance can not be achieved.
When a portion that has the fastest circumferential velocity of the backspin is on the equator, a dimple effect achieved by the golf ball with dimples arranged orderly is particularly small. On the other hand, when the portion that has the fastest circumferential velocity of the backspin is not on the equator, a measure of dimple effect can be achieved. In this golf ball, a flight distance depends on the rotation axis of the backspin. This golf ball is inferior in aerodynamic symmetry.
An object of the present invention is to provide a golf ball with which a great flight distance is achieved and that is excellent in aerodynamic symmetry.